1. Technical Field
This invention relates generally to battery charging systems, and more particularly to a battery charging system capable of protecting a battery cell despite the failure of any single component.
2. Background Art
Battery chargers are inherently complex systems. While some may think that all a battery charger does is “dump” current from a wall outlet into a rechargeable cell, nothing is farther from the truth. In addition to power conversion and filtering, charging systems offer safety protection to ensure that batteries are not overcharged. Some charging systems include other features like fuel gauging as well.
Safety is a very important issue for battery chargers. Common prior art battery chargers generally contain an AC-DC power converter, like a flyback power supply, and various serial voltage filtering and current limiting components that ensure the rechargeable battery is not overcharged. A common problem with these systems occurs when one of the serial components fails. For example, assume a battery charger includes an AC-DC converter (which converts 120V AC from the wall to 5V DC), and a serial current limiting circuit. If the current limiting circuit (which is often a transistor operating in its linear range) fails in a shorted condition, the battery may become overcharged, potentially venting combustible gasses.
The common solution to this component failure problem is to simply add redundant components. If there is one serial current regulator, add another. If there is one voltage regulator, add another. By doubling all safety components, two component failures are required to compromise the safety of the charger. The problem with doubling components, however, is cost. Doubling each of the components essentially doubles the overall cost of the charger.
There is thus a need for an improved battery charger that can sustain a component failure anywhere in the circuit without compromising charger reliability.